How Supervolcanoes Work

There are no universally accepted criteria for supervolcanoes. Volcanoes exist on a continuum, albeit a logarithmic one, so categorical edges tend to be fuzzy [sources: Achenbach; Geological Society of London; Tyson]. Consequently, estimates vary regarding the number of supervolcanoes, and of how often they blow their tops.

But a few common delineations do exist, including magnitude, the volume or mass of magma erupted, and intensity, the rate at which that magma erupts [source: Geological Society of London]. Magma is hot, molten material that comes from within Earth's mantle or crust and is forced out as lava during volcanic eruptions. It typically contains silicates, suspended crystals and dissolved gases [source: Oxford Dictionary of Science].

Another common categorization, called the Volcano Explosivity Index (VEI), classes volcanoes according to ash column height and the quantity of ash, pumice and lava ejected [source: USGS]. Supervolcanoes commonly occupy the highest VEI category, magnitude 8, meaning they produce more than 240 cubic miles (1,000 cubic kilometers) of erupted material and a plume more than 16 miles (25 kilometers) high [sources: Marshall; Rowlett; USGS]. Supervolcanoes rain destruction upon entire regions and leave behind calderas the size of Rhode Island [sources: Achenbach; Geological Society of London; Robinson; Tyson].

For such enormous and destructive wonders, supervolcanoes are surprisingly difficult to spot. Indeed, their size and power are part of the problem. Instead of building mountains, these giants blow them up. In fact, the Yellowstone National Park supervolcano was discovered partly due to a gap it created in the otherwise rugged landscape. Even then, its sheer extent -- 30 by 45 miles (50 by 70 kilometers) -- beggars the mind's ability to take it all in [sources: Achenbach; Geological Society of London; Tyson].

Add to that the vastness of time -- the hundreds of thousands to millions of years in which a caldera can erode, fill with lava from smaller eruptions or become a tree-lined lake -- and it's not hard to understand how supervolcanoes can hide in plain sight. But researchers remain stymied by yet another immensity, namely the scale of the processes that feed them -- mechanisms that reach deep into Earth and extend hundreds of miles across [sources: Friedman-Rudovsky; Geological Society of London; Marshall; Tyson; USGS].

So don't think of them as scaled-up volcanoes. Supervolcanoes are a phenomenon all their own, a deep process that we still struggle to comprehend [sources: Achenbach; Malfait et al.]. To better understand how they work, researchers turn to potential supervolcanoes like Uturuncu in Bolivia, which has grown half an inch (1.3 centimeters) per year for the past two decades, and to known hot spots of upwelling magma like the one beneath Yellowstone [source: Friedman-Rudovsky].

Trace a line on the map from northern Nevada through southern Idaho and up into northwest Wyoming, and you'll follow an intermittent scar of volcanic mayhem extending 350 miles (560 kilometers) and stretching back 18 million years. The chain of volcanoes grows progressively younger as you move along this west-to east line, each one marking an area where magma pressure from a lone hot spot broke through. The chain, like the hot spot, dead-ends in Yellowstone National Park [sources: Achenbach; Geological Society of London].

Actually, it isn't the hot spot that moves. Rather, the North American plate grinds overhead at around 1.8 inches (4.6 centimeters) per year. Every so often, the hot spot bursts through. During its more than 2 million years beneath Yellowstone, it has produced three jumbo-sized events [sources: Achenbach; Robinson; Tyson; USGS]:

Today, the Yellowstone hot spot has assumed a mellower mien, as far as we know. It heats the park's famed geysers, hot springs, steam vents and mudpots, and takes some of the chill off of Yellowstone Lake -- itself partly formed from a collapsed supervolcano caldera. But it also occasionally causes the ground above to dome unsettlingly and remind us that a sleeping dragon is still, after all, a dragon [sources: Achenbach; Encyclopedia Britannica; USGS].

Although researchers monitor Yellowstone for earthquakes, ground deformation, stream flow and temperature, how much warning a supervolcano might provide before erupting remains unclear [sources: Geological Society of London; Tyson; USGS]. Earthquakes, of which Yellowstone has 1,000-3,000 annually, might warn of a volcanic event, but they might also release pressure and thereby help prevent one [sources: Achenbach; USGS].

Supervolcanoes also periodically release pressure via smaller eruptions. In the 640,000 years since Lava Creek, Yellowstone has experienced roughly 80 nonexplosive, lava-producing eruptions, and the next Yellowstone eruption will more likely be of the scale of Pinatubo -- far from insignificant, but not a supervolcano [sources: Achenbach; USGS].

But what if the dice don't roll our way? What might a supervolcanic eruption at Yellowstone be like?

Most supereruptions occur in areas that remain active over millions of years but enjoy a long repose period, so don't place too much confidence in Yellowstone's apparent calm. Broadly speaking, the longer the dormancy, the bigger the boom [source: Geological Society of London].

Like other supervolcanic areas, Yellowstone sits on a long-active tectonic zone, a weakened and thinned crust overlying a 2,500 F (1,370 C) magma dome rising from the upper mantle. This dome has melted and broken into the crust to create two magma chambers roughly 5-7 miles (8-11 kilometers) underground, each measuring 30-plus miles (48-plus kilometers) across [source: Encyclopedia Britannica]. These magma chambers are filled with an amalgam of magma, semisolid rock and dissolved gases like water vapor and carbon dioxide.

Over centuries and millennia, additional magma builds up, delivering more heat and pressure, pushing overlaying ground upward little by little. If the chamber receives a steady and substantial supply of hot magma, pressure builds in an often cyclical process called incubation. If it doesn't, then some material solidifies and sinks, removing pressure. The sheer volume of a supervolcano's magma chamber means that incubation requires a heat delivery rate 2-3 orders of magnitude greater than that of a traditional volcano [sources: Achenbach; Klemetti].

Eventually, overpressure creates fractures along the dome's periphery, venting pressure from the chamber. The gas-filled magma blasts skyward, raining ash and debris over hundreds of miles and releasing deadly pyroclastic flows -- fast-moving, thick clouds of gas, ash and rocks boiling away from the eruption at 1,470 F (800 C) – across tens of thousands of square miles [sources: Achenbach; Geological Society of London].

Additional blasts pop off periodically for weeks. Ash drifts down on a regional scale, filling the sky with pollutants and blanketing tens of millions of square miles in inches of crop-killing ash [sources: Geological Society of London; Klemetti]. Until it settles, anyone within thousands of miles around risks breathing tiny glass needles, bursting pulmonary blood vessels and drowning in a slurry of ash and lung moisture [sources: Achenbach; Geological Society of London; Tyson]. Ash collapses roofs, pollutes vital water sources and gums up vehicle engines, sparking a crisis of food production, transportation, communication and economics lasting months to years [sources: Geological Society of London; Klemetti].

Within weeks, dust and sulfate aerosols encircle the globe, filtering out sunlight and cooling global average temperatures by an estimated 5-9 F (3-5 C) for several years afterward [sources: Geological Society of London; Klemetti; Marshall]. One-third of the U.S., particularly the states of Montana, Idaho and Wyoming, remain uninhabitable for months, possibly years [sources: Tyson; USGS].

Thankfully, the odds argue against this happening any time soon. But another supereruption, someday, somewhere in the world, is inevitable. Maybe it's time we got started on that Mars colony after all.